1. Field of the Invention
The present invention generally relates to systems and methods for reducing alteration of a specimen during analysis for charged particle based and other measurement systems that involve direct or incidental impingement or ejection of electrons or ions, which might catalyze a surface contamination reaction or deposition or removal of a measured species. Certain embodiments relate to a system that includes an element, which is disposed within a vacuum chamber, and which has a surface that is cooled such that molecules in the vacuum chamber are adsorbed onto the surface.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polish, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
As the dimensions of semiconductor devices shrink, metrology and analysis processes used to measure characteristics of the semiconductor devices become increasingly important. For instance, the acceptable variation in the characteristics of the features of the semiconductor devices (e.g., critical dimension, “CD”) also decreases. Therefore, accuracy requirements of metrology and analysis processes and tools have become more stringent. In addition, as the dimensions of semiconductor devices shrink, the capability of metrology tools (e.g., resolution) have been increased. In many instances, previously sufficient technologies (e.g., optical microscopy) have been replaced with more advanced and complex technologies including charged particle beam systems (e.g., scanning electron microscopy, “SEM”).
There are, however, some disadvantages to using charged particle beam based inspection, metrology, and/or analysis tools for semiconductor applications. For instance, in the presence of an electron beam, molecules present in the vacuum chamber of the tool or on the surface of the specimen may react with the specimen itself. Such reactions may result in the formation of a foreign material on the specimen such as a carbon containing material. Other reactions may result in the reduction or removal of one or more elements or molecules from the specimen.
Results of analysis of a specimen performed by charged particle beam based tools may be affected by such reactions. For instance, in the case of a reaction which produces a material on the surface of the specimen, a metrology tool may produce a measurement that reflects the dimensions of the specimen after the formation of the material, not the dimensions of the specimen as produced by the semiconductor manufacturing process. In the case of a reaction that removes material from the specimen, an analysis tool may produce a measurement that reflects the composition of the specimen after the removal of the material, not the composition of the specimen as produced by the semiconductor manufacturing process, or not the composition of a foreign particle being analyzed in a SEM based review system.
The reactions described above, therefore, reduce the accuracy of the charged particle beam based tools. In addition, the reactions described above may reduce the precision of the charged particle beam based tools. As a result, any process monitoring and control carried out based on the results of these tools may not be effective at maintaining or increasing the yield of the semiconductor manufacturing process. In some instances, in fact, process monitoring and control performed using results produced by these tools may actually be counterproductive resulting in decreased yield.
Possibly more important, however, are the changes to the specimen caused by the reactions that take place in charged particle beam based tools. In particular, it is often advantageous to perform inspection, metrology, and analysis of product wafers. Product wafers can be generally defined as wafers on which devices are actually formed, as opposed to monitor wafers, which may only be processed through a limited number of manufacturing processes to monitor a subset of semiconductor manufacturing processes. In this manner, devices are not actually formed on monitor wafers, and instead monitor wafers are usually reworked or scrapped after inspection, measurement, or analysis. Inspection, measurement, or analysis of product wafers is, therefore, generally preferred since the results may more accurately reflect the actual characteristics of other product wafers.
However, any change to the product wafer caused by reactions that take place in charged particle beam based tools may alter the characteristics of the product wafer to such a degree that device performance, speed, and/or device longevity may be adversely affected. In a worst case scenario, the devices can no longer be successfully formed on the product wafer. Obviously, such effects on product wafers are highly undesirable due to the cost involved in producing product wafers. In addition, even if devices can be successfully manufactured on the altered product wafer, the alterations may adversely affect the bin yield, and therefore the profitability, of the semiconductor manufacturing process.
Attempts have been made to reduce the effects of processes performed by charged particle beam based tools on specimens such as wafers. One such approach involves increasing the turbo pumping performed on the analysis chamber in which inspection, metrology, or analysis of the specimen is carried out and/or the load lock chamber in which the specimen is located before introduction to the analysis chamber. However, such an approach may have limitations in that performance will be determined by the size and type of pump, conductance of the pump to the chamber, and the time that the wafer can remain in the load lock chamber. In addition, the source of reactant molecules may be in the analysis chamber itself, in which case additional pumping in the load lock will have no effect on reducing undesirable reactions on the surface of the specimen.
Another approach that may be used alone or in combination with increased turbo pumping is to heat the specimen either before or during the process performed by the charged particle beam based tool. Such an approach may be disadvantageous for a number of reasons. For instance, it may be disadvantageous to heat some types of specimens on which materials are present (such as resist or low K dielectrics) that may be altered by such heating. In addition, such heating of the specimen may reduce throughput of the tool since time is required for the temperature of the specimen to stabilize before any measurements are performed.
Accordingly, it may be advantageous to develop systems and methods for reducing alteration of a specimen during analysis by substantially preventing any reactions from taking place in charged particle beam based tools thereby substantially preventing contamination of the specimen, substantially increasing the accuracy and/or precision of the tools, substantially increasing the effectiveness of process control and monitoring performed using results of the analysis, or some combination thereof without substantially changing the temperature of the specimen.